The present invention relates to novel high strength polyethylene fibers and their applications. More particularly, it relates to high strength polyethylene fibers which can be widely used in various fields for industry, for example, as the chopped fiber fibers or staples to produce non-woven fabrics or spun yarns; as the ropes or nets for industrial or private use; as the materials for high performance textiles such as ballistic materials or items, or protective gloves; or as the reinforcing fibers for composite materials such as fiber reinforced concrete products or helmets.
For high strength polyethylene fibers, there have been disclosed, for example, in JP-B 60-47922, high strength, high modulus fibers produced by the xe2x80x9cgel spinning methodxe2x80x9d using ultrahigh molecular weight polyethylene as the base material. These high strength polyethylene fibers have already been widely used in various fields for industry, for example, as the ropes or nets for industrial or private use; as high performance textiles such as ballistic materials or items, or protective gloves; or as the geo-textiles or working nets in the filed of civil engineering and architecture.
In recent years, these high strength polyethylene fibers have been required to have further improved performance, particularly durability, for example, mechanical durability over a long period or adaptability under severe conditions in use. Even textiles such as sportswears, or fishing lines have also been required to have durability when used for a long period. In addition, reinforcing sheets or strands to provide earthquake resistance have been required to have durability, particularly flexural fatigue resistance or abrasion resistance, such that when wound around pillars or other parts they cause no occurrence of fiber breaking at the corners.
The high strength polyethylene fibers have excellent tensile strength and excellent Young""s modulus indeed, but on the other hand, the structure of their highly-oriented molecular chains is responsible for the drawback that they have poor durability, particularly poor flexural fatigue resistance and poor abrasion resistance, for example, as compared with polyesters or nylons for ordinary garments. Such drawback has become some obstacle to the wide application of high strength polyethylene fibers in various fields for industry.
Further, many attempts have been made to use high strength polyethylene fibers in the chemical processes, for example, application to non-woven fabrics such as chemical filters or battery cell separators, because of their excellent resistance to chemicals, light and weather or to apply high strength polyethylene fibers to reinforcing fibers for concrete or cement, because there has been a demand for fiber reinforced concrete products having high crack resistance and high toughness, as well as excellent impact resistance and excellent long-term durability, since accidents were caused by wall materials coming off or falling from the surface of railroad tunnels or bridges.
However, when chopped fibers or staples are produced by cutting the conventional high strength polyethylene fibers, fibrillation of the fibers or their high surface hardness is responsible for the drawback that these fibers get stuck together by pressure to form a bundle of fibers, lacking in dispersibility. Further, when they are used as the reinforcing fibers for concrete or cement, their dispersibility in the cement matrix becomes deteriorated by flexure or entanglement of the fibers. For this reason, various treatments have been needed, for example, premixing with cement, hydrophilicity-providing treatment using metal oxides, or binding with resins.
To overcome such drawbacks, the orientation of the extended polyethylene molecular chains should be more relaxed, which method, however, causes a lowering of strength and Young""s modulus and cannot, therefore, be employed. Further, polyethylene fibers have no strong interaction between the molecular chains and easily cause fibrillation by repeated fatigue, which makes it very difficult to improve the durability of these fibers.
Thus an objective of the present invention is to provide high strength polyethylene fibers and their applications, which fibers have about the same or higher strength and Young""s modulus than those of the conventional high strength polyethylene fibers, and further have excellent flexural fatigue resistance and excellent abrasion resistance, and hardly cause fibrillation, and still further have high surface hardness.
That is, the present invention relates to high strength polyethylene fibers characterized in that: the fiber comprises a high molecular weight polyethylene consisting essentially of a repeating unit of ethylene; it has an intrinsic viscosity number of 5 or larger and an average strength of 22 cN/-dtex or higher; and the measurement of the fiber by differential scanning calorimetry (DSC) exhibits a temperature-increasing DCS curve having at least one endothermic peak over a temperature region of 140xc2x0 C. to 148xc2x0 C. (on the low temperature side) and at least one endothermic peak over a temperature region of 148xc2x0 C. or higher (on the high temperature side).
The present invention further relates to high strength polyethylene fibers characterized in that: the fiber comprises a high molecular weight polyethylene consisting essentially of a repeating unit of ethylene; it has an intrinsic viscosity number of 5 or larger and an average strength of 22 cN/-dtex or higher; and the number of frictions until the fiber is broken in an abrasion test according to method B for measuring abrasion resistance in the Testing Methods for Spun Yarn (JIS L 1095) is 100,000 or larger.
The present invention still further relates to chopped fibers, ropes, nets, ballistic materials or items, protective gloves, fiber reinforced concrete products, helmets, and other products obtained from the above high strength polyethylene fibers.